[C++] Add pause capability to async task scheduler and support pause producing in new scan node

[asfimport]

When the scanner is asked to pause producing we should be able to simply pause the async task scheduler. However, the task scheduler does not have a pause capability at the moment.

When the async task scheduler is asked to pause it should simply stop submitting new tasks. There is no need to worry about the tasks in progress.

Reporter: Weston Pace / @westonpace
Watchers: Rok Mihevc / @rok

_{Note: This issue was originally created as ARROW-17755. Please see the migration documentation for further details.}